Aluminium-magnesium alloy product

ABSTRACT

Method for making an aluminium-magnesium alloy in the form of a rolled product, having the composition in weight percent of: Mg 4.8-5.6, Mn 0.05-0.4, Zn 0.40-0.6, Cu 0.06-0.35, Cr 0.25 max., Fe 0.35 max., Si 0.25 max., Zr 0.12 max., Ti 0.3 max., others (each) max. 0.05, (total) max. 0.15, and balance aluminium.

The invention relates to an aluminium alloy product in the form of arolled product or an extrusion. In another aspect, the invention relatesto a welded structure, comprising such an alloy product.

Aluminium-magnesium alloy products are known to be used in the form ofsheets or plates or extrusion in the construction of welded or joinedstructures such as marine and automotive applications, storage tanks,pressure vessels, vessels for land or marine structure. Wrought productsare products that have been subjected to mechanical working by suchprocesses as rolling, extruding, or forging. Rolled products may have agauge typically to about 200 mm.

A known aluminium alloy having appropriate formability and weldability,is the Aluminium Association (AA)5454 alloy. Although the formabilityand weldability of the AA5454 alloy are sufficient for manyapplications, the alloy does not meet the desired higher strengthlevels. There is a constant drive toward down-gauging, for which a basicrequirement is to increase the strength. With such fairly low Mg-levelin the range of 2.4 to 3.0 wt. %, the alloy product is not susceptibleto intergranular corrosion (“IGC”).

The aluminium alloy AA5083, which has a Mg content in the range of 4.0to 4.9 wt. %, having a higher strength level than AA5454, is known to besusceptible to IGC. This susceptibility to IGC is highly undesirable,because an alloy product that has low resistance against IGC cannot beused always in a reliable manner, in particular at service temperaturesabove 65° C.

The aluminium alloy AA5059, which has a Mg content in the range of5.0-6.0 wt. %, a Mn content in the range of 0.6-1.2%, a Zn content inthe range of 0.4-1.5 wt. %, and a mandatory Zr addition in the range of0.05-0.25%, has an improved resistance to amongst others IGC, andprovides a high strength also in the welded condition.

In spite of these references, there is still a great need for animproved aluminium alloy product having improved balance of strength,high formability and a good corrosion resistance, in particular againstIGC.

It is an object of the present invention to provide an Al—Mg alloysheet, plate or extrusion with improved formability as compared to thoseof the standard AA5083 alloy in the same temper. It is another object ofthe present invention to provide alloy sheets, plates or extrusionswhich can offer IGC resistance at least equivalent or better to those ofAA5083, in combination with an elongation A50 of 24% or more. It isanother object of the present invention to provide a method ofmanufacturing such alloy products.

According to the invention in one aspect there is provided analuminium-magnesium alloy in the form of a rolled product or anextrusion, having the composition in weight percent:—

Mg  4.5-5.6 Mn 0.05-0.4 Zn 0.40-0.8 Cu 0.06-0.35 Cr 0.25 max. Fe 0.35max. Si 0.25 max. Zr 0.12 max. Ti  0.3 max.

-   -   others (each) max. 0.05, (total) max. 0.15    -   balance aluminium.

By the invention can be provided an alloy product in the form of rolledproduct, sheet or plate, or extrusion that has a higher formability thanAA5083 when using the same or similar temper material.

Surprisingly, the alloy product according to the invention has goodresistance against corrosion, in particular against IGC. It has beenthought in the past that resistance against IGC is normally reduced whenthe Mg content exceeds about 3.0 wt. %, but the resistance against IGCof the alloy product according to the invention is high compared to mostconventional AA5000-series alloy products with a Mg content of more than4 wt. %. It has been found that the alloy product according to theinvention has a weight loss of less than 25 mg/cm² when tested aftersensitising at a temperature of 100° C. during 100 hours in accordancewith ASTM G67, and has a weight loss of less than 15 mg/cm² when testedafter sensitising at a temperature of 85° C. during 100 hours inaccordance with ASTM G67, resulting in that the alloy product may beused at a service temperature of 65° C. or more without any problems,e.g. typically at a service temperature of 80 to 100° C.

It is believed that the improved balance of properties available withthe invention, particularly the higher strength and good formability incombination with the improved corrosion resistance, in particularagainst IGC, results from the balanced combination of the alloyingelements Mg, Mn, Zn, and Cu in the given ranges. Particularly, it isbelieved that the Cu and Zn contents in the ranges according to theinvention at such relatively high Mg levels optimise the resistanceagainst corrosion, in particular the resistance against IGC andexfoliation corrosion, whereas the Mg and Mn contents in the givenranges optimise strength and formability of the alloy product.

Magnesium is the primary strengthening element in the alloy product. Mglevels above 4.5 wt. % do provide the required strength. The amount ofMg should not exceed 5.6 wt. %, in order to ensure an acceptablecorrosion performance and workability, e.g. by means of rolling, of thealloy product as such high Mg levels. Preferably, the Mg content in thealloy product is more than 4.8 wt. % by which the alloy product isprovided with a better optimised balance of tensile strength, yieldstrength, formability as measured by its elongation (A50), and itscorrosion resistance.

Manganese is an essential additive element also. In combination with Mg,Mn provides the strength and formability in the alloy product as well asin the welds of the alloy product. A preferred range for the Mn contentis 0.1 to 0.2 wt. %, and thereby providing a balance in providingsufficient grain size control and a good formability and in particularin achieving an elongation A50 of 24% or more in the final product.

Zinc is an important alloying element for achieving sufficient corrosionresistance in combination with a good formability of the alloy product.At least 0.40 wt. % Zn addition is required in order to achievesufficient resistance against IGC. It has been found that for this alloythat at a Zn content above 0.8 wt. %, the uniform elongation issignificantly reduced and thereby adversely affecting the formability ofthe alloy product, e.g. the reverse bendability is adversely affected.Preferably, the amount of Zn does not exceed 0.75 wt. %, and it is morepreferred that the content of Zn does not exceed 0.6 wt. %, in order tooptimise the balance of desired characteristics of the alloy product,and to further optimise the uniform elongation. The most preferred rangefor the Zn addition is in the range of 0.4 to 0.6 wt. %.

Surprisingly, in a narrow range copper has been found to increase theresistance against IGC even though the Mg content is relatively high.Normally in the art, a deliberate Cu addition is avoided in alloys ofthis type, since it is thought to harm the resistance against corrosion.When Cu is present above 0.06 wt. % in combination with the zinc, apositive effect has been found on the resistance against IGC. However,Cu should be kept below 0.35 wt. % in order to avoid an adverse effecton the resistance against corrosion, in particular in the resistanceagainst pitting corrosion. In an embodiment, the lower limit of Cu ismore than 0.075 wt. %, and more preferably more than 0.10 wt. %.Herewith a good resistance against IGC is better save guarded.Preferably, the amount of Cu does not exceed 0.24 wt. %. Herewith thebalance of desired characteristics is better achieved. More preferably,the amount of Cu not exceeding 0.18 wt. %, in order to preserve thecorrosion resistance in a weld zone also. It is more preferred if Cudoes not exceed 0.15 wt. %, to better ensure good corrosion resistancein a weld zone. Also, the general resistance against IGC in the alloyproduct is optimised.

Fe is not an essential alloying element, and tends to form for exampleAl—Fe—Mn compounds during casting, thereby limiting the beneficialeffects of Mn. Therefore Fe must not be present in an amount of 0.35 wt.% or more. For the mechanical properties of the product, in particularto improve the formability of the alloy product, the amount of Fe ispreferably to be kept below 0.2 wt. %.

Si is not an essential alloying element. It also combines with Fe toform coarse Al—Fe—Si phase particles which can affect the fatigue lifeand fracture toughness of for example the welded joints of the alloyproduct. For this reason, the Si level is kept to a maximum of 0.25 wt.%. Preferably the amount of Si is kept to a maximum of 0.2 wt. % andmore preferably of 0.12 wt. %, and most preferably at a maximum of 0.1wt. % in order to better ensure favourable formability characteristicsof the alloy product.

Zirconium is not essential for achieving the improved corrosionperformance in the alloy product according to the invention, but it canhave an effect to achieve a more fine grain refined structure in thefusion zone of welded joints. Zr levels of 0.15 wt. % or more are to beavoided, and should be less than 0.12 wt. %, since this tends to resultin very coarse needle-shaped primary particles with decrease in ease offabrication of the alloy product and in the formability of the alloyproduct. Zr may cause to form undesirable coarse primaries, inparticular together with Ti. In a preferred embodiment, the amount of Zrdoes therefore not exceed 0.05 wt. %. Moreover, it may be favourable tokeep Zr out of scrap source material for specific recycling reasons. Tothis extend, it is more preferred to limit the presence of Zr to lessthan 0.02 wt. %.

Titanium is often used as a grain refiner during solidification of bothcast ingots and welded joints produced using the alloy product of theinvention. This effect is obtained with a Ti content of less than 0.3wt. %, and preferably less than 0.15 wt. %. Ti may be replaced in partor in whole by V in the same compositional range to achieve a similareffect.

Chromium is an optional alloying element, that may improve further thecorrosion resistance and strength of the alloy product. However, Crlimits the solubility of Mn and, if present, also that of Zr. Therefore,to avoid formation of undesirable coarse primaries, the Cr level mustnot be more than 0.25 wt. %. Preferably, the Cr is present in a range of0.06 to 0.2 wt. %, and more preferred range is 0.11 to 0.2 wt. %.

The balance is Al and inevitable impurities. Typically each impurityelement is present at 0.05% maximum and the total of impurities is 0.15%maximum.

The aluminium alloy in the form of a rolled product may be provided in awide range of gauges, for example up to 200 mm, but a preferred gaugefor the alloy product according to the invention is in the range of 0.5to 5 mm.

The alloy product according to the invention can be delivered in varioustemper conditions. However, for the group of applications for which thealloy product is ideally suited, preferably it should be a tempersimilar to a soft worked temper, also known in the art as an “O”-temper,or, in case of thin plates, a light “H”-strain hardened temper such asfor example H1111.

The invention further relates to a welded structure comprising at leastone section of the product according to one of the above describedembodiments. The alloy product according to one or more embodiments ofthe invention is eminently suitable for application in such a weldedstructure due to its excellent weldability, and its high strength in aweld zone in combination with its improved corrosion performance.

The invention further relates to a pressure vessel, in particular awelded pressure vessel, comprising a shell that comprises the rolledaluminium-magnesium alloy product as is described above. Due to theincreased strength, such pressure vessel can be down-gauged to have alower weight. Moreover, the corrosion properties can be improved. Thepressure vessel, e.g. for a braking system, according to this aspect ofthe invention can be used at a higher service temperature, in particularabove 65° C.

The alloy product in accordance with the invention may be employed alsovery successfully for automotive applications, in particular as bodypanels, and structural parts such as suspension systems and wheels.

In another aspect, the invention relates to a method of producing analuminium alloy product comprising the sequential processing steps:—

-   -   (i). providing an intermediate alloy product having a        composition according to the mentioned above and set forth in        the claims;    -   (ii). cold working the intermediate alloy product to a final        gauge to obtain an intermediate wrought product;    -   (iii). annealing the intermediate wrought product by heating the        product at a heating rate in the range of 2 to 200° C./sec,        holding the product at a soaking temperature in the range of 480        to 570° C. for a duration of up to 100 sec, followed by a        cooling at a cooling rate in the range of 10 to 500° C./sec to        below a temperature of 150° C.        By this method it is achieved that the positive influence of Cu        on the resistance against IGC is fully exploited. Although the        alloy product has good properties when other annealing schemes        are applied, it is believed that the positive influence of Cu on        the corrosion properties is in particular enhanced by the        annealing scheme of processing step (iii).

The aluminium alloy as described herein can be provided in process step(i) as an ingot or slab for fabrication into a suitable wrought productby casting techniques currently employed in the art for cast products,e.g. DC-casting, EMC-casting, EMS-casting. Slabs resulting fromcontinuous casting, e.g. belt casters or roll casters, may be used also.

In order to obtain an intermediate product suitable for cold working,preferably by means of cold rolling, the provided intermediate alloyproduct can be hot worked by means of hot rolling or hot rolling incombination with one or more forging steps.

The annealing scheme of processing step (iii) can be applied in acontinuous annealing facility. The required heating rates can beachieved, for example, by homogeneous heating by means of inductiveheating. This gives further improved mechanical properties in the sheetsor plates.

Particularly favourable results have been obtained in an embodiment ofthe method wherein the soaking temperature is in the range of between520 and 550° C.

The balance of characteristics of the alloy product produced by themethod is found to be better optimised in the embodiment wherein theproduct is held at the soaking temperature for a duration of up to 40sec.

In an embodiment of the method, the heating rate is at least 50° C./sec,and preferably at least 80° C./sec. Herewith, the balance between themechanical properties and the resistance against IGC has been found tobe more favourable. This is especially the case when the cooling rateafter soaking is at least 100° C./sec.

The invention will now be explained with reference to laboratoryexperiments.

Various slabs were cast having chemical compositions as shown in thefollowing Table 1, balance aluminium. Slab A corresponds to a standardAA5083 alloy, and Slabs B and C are according to the invention.

TABLE 1 compositions (in wt %) of the cast slabs (Balance Al andimpurities) Slab Inv. Mg Mn Zn Cu Cr Fe Si Zr Ti A No 4.5 0.50 0.030.005 0.10 0.31 0.16 0.001 0.015 B Yes 5.23 0.17 0.51 0.12 0.16 0.230.10 <0.01 0.02 C Yes 5.23 0.17 0.51 0.12 0.16 0.23 0.10 <0.01 0.02 D No5.36 0.50 0.50 0.12 0.15 0.20 0.11 <0.01 0.02

The processing of the slabs A and B comprised a homogenisation annealduring 10 hours at a temperature of 510° C., hot rolling whereby theexit temperature was about 330° C., followed by cold rolling with 60%cold reduction and finally soft annealing in batch anneal at atemperature of 330° C. during 1 hour. The processing of slabs C and Dwas identical to those of A and B, with the exception of the final softanneal, which was a continuous anneal for 10 sec. at 530° C. Finalgauges were 3 mm, and the plates were delivered in H111-temper.

These products were tensile tested according to EN 10002, and theresults for the parallel (∥) and perpendicular (⊥) directions are givenin Table 2.

TABLE 2 Tensile strength (“UTS”), 0.2% Proof strength (“PS”), Elongation(“A50”) Direction of UTS PS A50 Alloy Testing [MPa] [MPa] [%] A || 299149 19 (AA5083) ⊥ 293 147 21 B || 311 146 22 ⊥ 310 147 24 C || 317 15325 ⊥ 314 152 26 D || 332 166 23 ⊥ 332 163 23

The elongation A50 is considered to be a measure for the formability.The results in table 1 indicate that the formability of the alloys B andC is improved when compared to alloys A (AA5083) or D. This effect iscontributed to the lower amounts of Mn in alloys B and C.

The alloy products have been subjected to a weight loss test accordingto ASTM G67 after sensitising at 100° C. for a duration of 100 hours inH111-temper condition. Results are shown in Table 3.

TABLE 3 Weight loss (in mg/cm²) after sensitising. A B C D 100 hr at100° C. 36 17 13 20This indicates that the corrosion resistance of products B and C is muchbetter than of the standard AA5083 alloy (A). Product C is below 15mg/cm², which is according to ASTM-G67 the upper limit for a productquality not susceptible to IGC, and product B is already close to thislimit.

The resistance against IGC of product C in the presently usedsensitising conditions show an improvement over that of B, apparently byusing the continuous anneal the corrosion resistance of the product isimproved. It is expected that under more severe sensitising conditionsthe difference is more clearly visible.

Alloy products B and C were welded without any problem using TIG weldingunder standard conditions.

In an additional test series, the influence of Cu on the corrosionresistance was tested. Some additional slabs were cast having thechemical compositions as shown in the following table 4, balancealuminium.

The processing of the additional alloys was identical to the processingof alloy C, i.e. with a final soft anneal as a continuous anneal.

TABLE 4 composition (in wt %) of the additional cast slabs Slab Inv. MgMn Zn Cu Cr Fe Si Zr Ti E No 5.58 0.16 0.51 0.02 0.15 0.21 0.11 <0.010.02 F Yes 5.49 0.16 0.51 0.09 0.16 0.20 0.11 <0.01 0.02 G Yes 5.41 0.160.50 0.21 0.16 0.20 0.11 <0.01 0.02 H Yes 5.42 0.15 0.50 0.30 0.15 0.200.11 <0.01 0.02 J No 5.51 0.17 0.51 0.41 0.16 0.20 0.11 <0.01 0.02

The alloy products have been subjected to a weight loss test accordingto ASTM G67 after sensitising at 100° C. for a duration of 100 hours inH111 temper condition. The alloy products have also been subjected to anASSET test according to ASTM G66 after welding, followed by sensitisingat 100° C. for a duration of 100 hours. The weld was a TIG weld usingAA5183 as filler wire. Results are shown in table 5. The ASSET resultscorrespond to the Heat Affected Zone, because here the most severeattack is found.

TABLE 5 Weight Loss (in mg/cm²) and ASSET result after sensitising Alloy% Cu WL [mg/cm²] ASSET result in HAZ E 0.02 37 N F 0.09 21 PA C 0.12 13PA G 0.21 13 PB H 0.30 11 PB J 0.41 12 PCAccording to ASTM G67 the upper limit for a product quality notsusceptible to IGC is 15 mg/cm². In ASTM G66 the range to classify theresults is given, but limits for acceptable or not acceptable are notspecified. However, for a person skilled in the art, it is clear thatpitting A is still acceptable whereas pitting C in unacceptable. PittingB is for most applications still acceptable.

The results indicate that the resistance against IGC increases withincreasing Cu content, but at the same time the resistance againstpitting decreases. For a Cu level of 0.30 wt. % and lower, theresistance against pitting is acceptable or better than acceptable. Theweight loss is thought to measure below 15 mg/cm² when the Cu level isabove about 0.11 wt. %.

Based on these results it is concluded that the broadest operationalwindow is found with Cu levels between 0.06 and 0.35 wt. %. Preferablythe amount of Cu does not exceed 0.18 wt. % in order to preserve thecorrosion resistance in a weld zone.

1. Method of producing a wrought aluminium alloy rolled productcomprising the steps of: (i) providing an intermediate alloy producthaving a composition in weight percent consisting of: Mg  4.8-5.6 Mn0.05-0.4 Zn 0.40-0.6 Cu 0.06-0.35 Cr 0.25 max. Fe 0.35 max. Si 0.25 max.Zr 0.12 max. Ti  0.3 max.

impurities (each) max. 0.05, (total) max. 0.15 balance aluminium, (ii)cold working the intermediate alloy product to a final gauge to obtainan intermediate wrought product, (iii) annealing the intermediatewrought product by heating the product at a heating rate in the range of2 to 200° C./sec, holding the product at a soaking temperature in therange of 480 to 570° C. for a duration of up to 100 sec, followed by acooling rate in the range of 10 to 500° C./sec to below a temperature of150° C.
 2. The method according to claim 1, wherein the amount of Zrdoes not exceed 0.05 wt. %.
 3. The method according to claim 1, whereinthe amount of Zr does not exceed 0.02 wt. %.
 4. The method according toclaim 1, wherein the amount of Cu is more than 0.075 wt. %.
 5. Themethod according to claim 1, wherein the amount of Cu is more than 0.10wt. %.
 6. The method according to claim 1, wherein the amount of Cu doesnot exceed 0.24 wt. %.
 7. The method according to claim 1, wherein theamount of Cu does not exceed 0.15 wt. %.
 8. Method according to claim 1,wherein the soaking temperature during step (iii) is in the range of 520to 550° C.
 9. Method according to claim 1, wherein the heating rateduring step (iii) is at least 50° C./sec.
 10. Method according to claim9, wherein the heating rate during step (iii) is at least 80° C./sec.11. Method according to claim 1, wherein the annealing of step (iii) iscarried out in a continuous annealing facility.
 12. wherein the finalgauge of the aluminium product is at most 5 mm.
 13. Method according toclaim 1, wherein the final gauge of the aluminium product is in a rangeof 0.5 to 5 mm.
 14. Method according to claim 1, wherein the productafter annealing has a weight loss after sensitizing for 100 hours at100° C. of less than 25 mg/cm² when tested against intergranularcorrosion in accordance with ASTM G67.
 15. Method according to claim 1,wherein the product after annealing has a weight loss after sensitizingfor 100 hours at 85° C. of less than 15 mg/cm² when tested againstintergranular corrosion in accordance with ASTM G67.
 16. Methodaccording to claim 1, wherein the amount of Zn in the alloy product doesnot exceed 0.6 wt. %.
 17. Product according to claim 1, wherein theamount of Cr in the alloy product is in a range of 0.11 to 0.35 wt. %.18. Product according to claim 1, wherein the amount of Cr in the alloyproduct is in a range of 0.11 to 0.2 wt. %.
 19. Product according toclaim 1, wherein the amount of Fe in the alloy product is less than 0.2wt. %.
 20. Product according to claim 1, wherein the amount of Si in thealloy product is maximum 0.12 wt %.
 21. Product according to claim 1,wherein the amount of Si in the alloy product is maximum 0.1 wt. %. 22.Method according to claim 1, wherein the amount of Mg in the alloyproduct is at least 5%.
 23. Method according to claim 1, wherein theamount of Ti in the alloy product is less than 0.15 wt. %.
 24. Methodaccording to claim 1, wherein the amount of Mn in the alloy product isin the range of 0.05 to 0.2 wt %.
 25. Method according to claim 1,wherein the amount of Mn in the alloy product is in the range of 0.1 to0.2 wt. %.